1. Field of the Invention
The invention relates to a method and apparatus for evaluating the quality of a spot weld. More specifically, the invention relates to a method and apparatus for determining the size and quality of a nugget in a spot weld through analysis of ultrasonic waves directed into the weld.
2. Description of Related Art
A conventional spot weld inspecting device, as shown in FIG. 1, comprises a single ultrasonic probe 50, comprising a transducer 52 and a delay line 54 having a delay line surface 56 for direction against a surface being evaluated. The probe 50 transmits an ultrasonic wave into a weld and receives a wave reflected therefrom. The device includes a display that operates to receive a signal from the probe and display a visual representation of the reflected wave.
The structure of a typical spot weld is shown in FIG. 1. In this representation, two metal sheets 100, 200 are connected by a spot weld 300. The area of the spot weld can be classified into three general zones which the ultrasonic wave will intersect:
Zone 1 corresponds to an air gap 10 between the metal sheets;
Zone 2 is the weld nugget 20, the diameter of which is to be measured; and
Zone 3 is a stick weld area 30, wherein the sheets 100, 200 are surface bonded without the metallurgical structure formation found in the weld nugget 20.
The weld nugget 20, zone 2, and the stick weld area 30, zone 3, are substantially transparent to the ultrasonic waves. This is shown in FIG. 1 by the ultrasonic echo responses represented by rays 1, 2, 3. The ultrasonic waves are reflected at any boundary having a change in properties, such as the interface of a metal sheet and the surrounding air. Ray 1, corresponding to zone 1, is completely reflected from the inner face 120 of sheet 100 due to the air gap, whereas rays 2, 3 pass through zones 2, 3 and are not reflected until they reach the opposite face 220 of metal sheet 200.
Referring now to FIG. 2, pulses S11, S21, S31 are a graphical display of the ultrasonic waves represented by rays 1, 2, 3, respectively, as they are reflected to the ultrasonic probe 50 by a respective surface of the metal sheet 100, 200 in the area of the indentation mark 70. Pulse S11 is the first wave reflection of a given ultrasonic pulse transmitted by probe 50. Not all of the reflected pulse returns to the ultrasonic probe 50. Some is reflected back into the metal sheet by the near face 110 of metal sheet 100. Thus, multiple signals will be received for each wave transmitted. In FIG. 2, pulses S12, S13, S14 represent these multiple echoes inside the metal sheet 100. Each echo is time delayed, with the time between echoes T1=2 d1/C, where d1 is a thickness of the metal sheet 100 and C is the sound velocity within the material.
Likewise, pulses S22, S23, and S32, S33 represent subsequent reflections from the opposite face 220 of the spot weld in the area of the weld nugget 20 and stick weld 30 respectively. The periods of these pulses are T2=T3=2 (d1+d2)/C, where d2 is the thickness of the lower sheet. The amplitude of the signals S1, S2, S3 is a decreasing function of time due to attenuation of the signal as it continues to partially reflect within the metal sheets 100, 200. The rate of decay is slightly larger for S2 than for S3 because the grain structure of the nugget 20 results in a greater signal attenuation than is found in the stick weld area.
Thus, if there is a gap between sheets within the cross section of the ultrasound beam it can be easily detected in the time domain. It is only possible to distinguish nugget and stick zone by analysis of the slope of the pulse trains, i.e. the rate of decay of the signal echoes.
When a single transducer covers several zones, the output signal S is a sum of the signals S1, S2, S3 weighted by the factors A1, A2, A3:S=S1 A1+S2 A2+S3 A3
where A1, A2, A3 are areas of the of the spot weld zone with delamination (air gap 10), weld nugget 20 and stick weld 30, measured in a plane defined between the sheets 100, 200.
Signal S1 can be eliminated from this relationship by time discrimination. The signals S2 and S3 are overlapping in time domain, so it is difficult to estimate S2, i.e. nugget spot weld size. Practically, even a skilled operator can only distinguish the case of very good weld (A1+A3≈0) and the case of very bad weld (A2≈0).
The second general problem comes from the stochastic nature of the weld formation. The boundary between zones with high and low attenuation is smooth therefore it is difficult to predict the position where the weld material becomes strong enough to withstand applied mechanical force. Thus acoustically determined diameter is not necessary coincide with the nugget size obtained from the mechanical destructive test.
To separate responses S2 and S3 it was proposed to employ a ring-shaped probe (U.S. Pat. No. 4,208,917). The outside diameter of the probe is equal to the outside diameter of the electrode tip, and the inside diameter of the probe is equal to the minimum diameter of the weld nugget according to the Spot Welding Standard. Thus if the signal recorded by this probe demonstrates low attenuation it means that the sound travels through the stick weld zone and the nugget diameter is smaller than the minimum size.
Nevertheless the result of the weld test strongly depends upon where the probe was located relative to the weld nugget area. In addition the size of the nugget can not be measured with this technique.
The mechanical scanning systems with single probe (U.S. Pat. No. 4,265,119) or several probes (U.S. Pat. No. 6,116,090) employs the ultrasonic beam with diameter much smaller than the minimum nugget size. These systems are potentially able to detect the boundary between weld nugget 20 and the stick weld area 30 or the air gap 10. Nevertheless, its usage is not convenient for mass production non-destructive testing because of the time-consuming mechanical scanning procedure. In addition, it is necessary to apply an immersion liquid like water instead of an ultrasonic gel for sound coupling. This is difficult when conducting test procedures on tilted or vertical surfaces because of the high fluidity of the immersion liquid.
Electronic scanning using arrays of ultrasonic transducers allows overcoming the problems of the mechanical scanning. An example of an apparatus using this technique is described in U.S. Pat. No. 6,072,144. The disclosed phased array system can produce accurate weld nugget size measurement and employs an immersion water column design and a thin film to hold the immersion liquid. This film is in contact with the weld surface and can be easily destroyed by the roughness of the weld surface. Such limitations are not well suited for a production level test procedure. In addition, the large differential between the acoustic impedance of steel and the immersion liquid considerably reduces the effectiveness of phased arrays.
In the examples mentioned above, the surface of the spot weld is assumed to be a perfect flat surface. It is also assumed that the probes occupy stable, proper position on the metal sheet. However, most real spot welds have irregular curved surfaces on both sides, as shown in FIG. 1, and the metal sheets can be deformed in the vicinity of the spot weld. The real shape of the spot weld and the deformation of the metal sheets are related to the type of welding machine and electrodes, parameters of welding process, electrode tip wear, type and thickness of the welded material and other surface conditions. Thus, the probe can be randomly tilted so that the ultrasonic ray intersects the surfaces at different angles which may not be equal to 90°. This situation is illustrated in FIG. 3, where ray 1 intersects the upper surface of the indentation 70 at angle α and is reflected from the lower surface at angle β. Due the irregularity of each surface, the values of these angles will generally be different for every point on the spot weld.
The transmission and reflection coefficients of an ultrasonic ray are sensitive to the tilt of the surface. The amplitude of the response signal received from the particular point depends on the angles α, β. Thus, the amplitude decay is determined by the curvature of the indentation mark, and the attenuation of the sound waves inside the weld as well. The above-described method for nugget and stick weld size estimation, based on the attenuation rates, becomes incorrect when the surfaces are curved.
The gap between the delay line surface 56 and metal surface 110 causes an additional time shift of the reflected signals. Because the signal reflections from the air gap 10 are filtered out on a time-response basis, the varying gap between the probe and the surface 110 can cause confusion in determining the proper time delay. An initial reflection from the surface 110 can be mistaken for a reflection from air gap 10. This could be especially true for a thin sheet 100 and a deep indentation 70.
It would be advantageous to develop a non-destructive testing method of spot welds using an ultrasonic apparatus that displays a proper representation of the weld structure, that is also adaptable for use in a high volume production setting.